Process and device for manufacturing polyurethane foam moldings

ABSTRACT

A process and device are disclosed for manufacturing foam moldings. The parting plane of the mold has a circumferential vacuum channel, the top mold half has a suction channel for quickly evacuating the mold and self-closing expansion channels are arranged at the highest points of the inner cavity of the mold.

FIELD OF THE INVENTION

The present invention relates to a process and a device for theproduction of shaped foamed polyurethane articles by introducing anexpandable polyurethane reactive mixture into a mould divided by aparting plane, wherein the parting plane has a circumferential vacuumchannel through which the mould cavity defined by the mould halves isevacuated. There have already been a number of proposals for this; seee.g. DE-A 15 04 278, DE-A 30 20 793 and EP-A 23 749.

BACKGROUND OF THE INVENTION

Evacuation of the mould is necessary in particular in order to removethe gas present in the mould from the mould to avoid the formation ofvoids. To achieve this, it is necessary to construct the mould halvessuch that the parting plane lies at the highest point of the mouldcavity, since otherwise a pocket forms from which the gas present can nolonger can be drawn off. This could be counteracted by evacuating themould cavity to a very low pressure of below 100 mbar, in particularbelow 50 mbar, before the foam rises, i.e. before the level of the foamin the mould cavity passes beyond the parting plane. However, such a lowpressure in the mould cavity means that the foam initially expands sorapidly and severely, before substantial contents of blowing agent haveyet been released, that only an irregular foam structure forms.

Another disadvantage of evacuation via the parting plane is that thedelivery via the parting plane is relatively low, especially if theexpandable reactive mixture is introduced into the open mould andevacuation is carried out only after the mould has been closed, so thatvery long cycle times are necessary.

The properties of polyurethane foam are substantially determined by thebulk density of the finished foam and the material properties of thematrix. If water is employed as a chemical blowing agent in particular,in which case carbon dioxide is liberated by the reaction of the waterwith the isocyanate, the recipe of the expandable polyurethane reactivemixture must be adjusted precisely to establish the matrix properties.It is therefore in principle desirable to be able to produce foams ofdifferent bulk densities with a single recipe. Controlling the pressurein the foam mould is outstandingly suitable for controlling the bulkdensity of a foam while simultaneously retaining the recipe, especiallyof the blowing agent content, see e.g. EP-A 23 749 and EP-A 44 226.

SUMMARY OF THE INVENTION

The object of the present invention is to produce, over short cycle orsequence times, void-free mouldings of lower bulk density than is to beexpected on the basis of the blowing agent content under atmosphericpressure, it also being possible to employ contoured mould cavities inwhich the parting plane cannot be placed at the points of maximum heightof the mould cavity for geometric reasons.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a device according to the invention.

FIG. 2 shows the preferred embodiment of the expansion channels.

FIG. 2a shows the preferred embodiment of the expansion channels of FIG.2 along line A—A.

FIG. 3 shows the pressure course with respect to time, according to theinvention, in the mold.

DETAILED DESCRIPTION OF THE INVENTION

According to the invention, this is achieved in that the upper mouldhalf has a closable suction opening through which the internal pressurein the mould is evacuated rapidly, preferably within the cream time ofthe expanding polyurethane reactive mixture, to 300 to 800 mbarabsolute, in that the suction opening is closed when the given internalpressure in the mould is reached, wherein the expandable mixture expandsto fill the mould cavity, expansion channels with such a lowcross-section that they are closed by penetration of the expandingmixture are provided at the point or points of maximum height of themould cavity, and the parting plane has a vacuum seal to avoidpenetration of gas, the suction opening, the vacuum channels and theexpansion channels being connected to the same vacuum source.

The mould cavity is preferably evacuated to 400 to 700 mbar absolute.

The increase in pressure after the foam has risen, i.e. substantiallyafter all the blowing agent has been released, takes place due to anincrease in temperature because of the heat of reaction and optionallysmall further amounts of blowing agent released. It should preferablytake place to a pressure of 700 to 1,100 mbar absolute, particularlypreferably 700 to 900 mbar.

According to the invention, the expandable polyurethane reactive mixturecan be introduced into the open mould without disadvantage to the cycletime, since the desired minimum internal pressure in the mould to whichthe expanding foam can be exposed without disadvantage is achievedwithin the shortest time through the suction opening provided accordingto the invention.

The present invention also provides a device for the production ofshaped foamed polyurethane articles which has a mould cavity defined byan upper and lower mould half, which can be separated by a partingplane, and means for introducing an expandable polyurethane reactivemixture into the lower mould half, wherein the parting plane has avacuum seal, the upper mould half has a closable suction opening and thepoints of maximum height of the upper mould half have expansionchannels, the cross-section of which is such that they can be closed bypenetration of reactive mixture which has not yet cured, and a commonvacuum source and connecting lines from the vacuum source to the vacuumseal, suction opening and expansion channels.

The vacuum source is preferably constructed in the form of a vacuum tankseveral times the volume of the mould cavity, the vacuum tank beingconnected to a vacuum pump.

The expansion channels can have a circular or slit-like cross-section.The lower cross-section dimension of the expansion channels ispreferably 0.1 to 0.5 mm, particularly preferably 0.1 to 0.3 mm.

The expansion channels are particularly preferably equipped with anejection ram, for example needle-shaped, for the polyurethane which haspenetrated. The expansion channels are furthermore preferably formed bygaps between an ejection ram of larger dimensions and its bore throughthe upper mould half.

The invention is described in more detail below with the aid of theattached figures. FIG. 1 shows a device according to the invention.

FIG. 1 shows a mould 1 which comprises a lower mould half 2 and an uppermould half 3, wherein the mould halves are separated by the partingplane 4. A circumferential channel 5 is provided as a vacuum seal in theparting plane. The upper mould half furthermore has a suction opening 6which can be closed, for example, by the closing cone 11 when thedesired internal pressure in the mould is reached. Expansion channels 7a and 7 b are furthermore provided at the points of maximum height ofthe interior of the mould. The vacuum seal 5, suction opening 6 andexpansion channels 7 a and 7 b are connected via corresponding lines 10to the vacuum tank 8, shown in reduced size in the diagram, from whichgas is sucked via the vacuum pump 9.

FIG. 2 shows in section the section of an upper mould half 3 with themaximum height of the interior of the mould. The expansion channel 7 isconstructed in the form of flattened areas of the ram 27 which can beejected by means of a hydraulic device 18. The expansion channel here isstaggered in sections over the circumference of the ejection ram 27, thestaggered expansion channel sections being connected to one another viaa circumferential groove 17. During rising of the foam 20, gas isfurther drawn off from the interior of the mould through the expansionchannels and via the connecting line 10 to the vacuum source, so thatthe low internal pressure in the mould is maintained. When the expandingfoam reaches the expansion channels 7, it penetrates into these, closingthese at the same time. After the foamed moulding is removed from themould, the ejection ram 27 is moved out of the casing 19 by means of thehydraulics 18 so that the polyurethane which has penetrated into theexpansion channels 7 can be removed.

FIG. 3 shows by way of example the course of the pressure within themould in a conventional process (I) and a process according to theinvention (II) for foam moulding production.

The pressure passes through the following phases here:

A: Start of the introduction of the expandable polyurethane reactivemixture into the open mould.

B: Introduction has ended; start of the cream time; closing of themould.

C: Mould is closed, rapid drop in pressure due to evacuation via thesuction opening (according to the invention)

D: The desired internal pressure in the mould of 500 mbar absolute hereis reached; transition from the cream time to the rising time.

E: The foam fills the mould; end of the rising time; start of theholding pressure time. In the conventional foam moulding process (I), anoticeable increase in pressure is already to be detected within themould because of the resistance of the displaced gas due to the risingfoam.

F: The maximum internal pressure of the mould, which corresponds to themaximum internal pressure of the foam cells, is reached.

G: Opening of the mould and removal of the moulding.

What is claimed is:
 1. A process for the production of shaped foamedpolyurethane articles by introducing an expandable polyurethane reactivemixture into the lower mould half of a mould divided by a parting planeand evacuation of the mould, the parting plane having a vacuum seal,characterized in that the mould cavity is evacuated rapidly through aclosable suction opening provided in the upper mould half to a pressureof 300 to 700 mbar absolute, the evacuation pressure which is a functionof the recipe of the expandable polyurethane reactive mixture is suchthat the final internal pressure in the mould is 800 to 1,100 mbar, thesuction opening is closed, the reaction mixture expanding to fill themould cavity and the expanding mixture penetrating into expansionchannels which are provided at the point or points of maximum height ofthe mould cavity and have a low cross-section so that said expansionchannels close, the vacuum seal, suction opening and expansion channelsbeing connected to the same vacuum source.
 2. A device for theproduction of shaped foamed polyurethane articles comprising a moldcavity defined by an upper mold half and a lower mold half which cavitycan be separated by a parting plane and the parting plane having avacuum seal, and means for introducing an expandable reactive mixtureinto the lower mold half, which is characterized in that the upper moldhalf has a closable suction opening and, at the points of maximum heightof the interior of the mold, has expansion channels which are formed bygaps between an ejection ram passing through the upper mold half and itsbore, the cross-section of these channels is such that they can beclosed by penetration of reactive mixture which has not yet cured, and avacuum source and connecting lines from the vacuum source to the vacuumseal, suction opening and the expansion channels.
 3. The device of claim2 in which the vacuum source is constructed in the form of a vacuum tankseveral times the volume of the mold cavity, and the vacuum source isconnected to a vacuum pump.